This invention relates to devices that respond to fluid pressure by bending. This invention is more specifically concerned with an elongated flexible probe having a distal steering section, and in which the steering section is of the fluid bendable type. The invention is also concerned with a main insertion tube construction that compensates for axial stretching of the bending neck.
A borescope, endoscope, or similar flexible probe can be generally configured as an elongated flexible insertion tube with a viewing head at its distal or forward end and a control housing for controlling or steering the distal or forward end. The typical borescope has a bendable tubular steering section or articulation section at the distal end adjacent the viewing head. The steering section typically comprises a series of alternating wobble washers and spacers, with control cables that extend through the wobble washers and then through the remainder of the flexible insertion tube. The steering cables connect with a steering control unit in the control section. Each such pair of cables is differentially displaced to bend the steering section in a bending plane. In this manner, the viewing head can be remotely oriented to facilitate the inspection of an object. Borescopes are often required to bend in narrow, tortuous passageways, so the radial dimension of the borescope is often quite limited, i.e., 6 mm diameter. Also, the pathway to the object or target can be quite long, which then requires the insertion tube and the steering cables to be rather long, e.g., fifteen feet or more.
A number of cable-actuated articulation or steering mechanisms are known, and typical ones are discussed in U.S. Pat. Nos. 3,610,231; 3,739,770; 3,583,393; 3,669,098; 3,779,151; and 4,347,837. Another steering mechanism is described in U.S. Pat. No. 4,700,693.
These cable-actuated articulation mechanisms require the cables to have a significant amount of slack or play because bends and coils in the insertion tube effectively shorten the cables and because the articulation section bends at discrete points rather than following a smooth arc. However, in many applications, such as jet engines, the articulation section must bend rather precisely to penetrate the tortuous passages into the area to be inspected without damaging delicate parts. For these reasons, cable tension must be limited and cable slack must be minimized. Moreover, where the insertion tube is long, extra cable slack is often included to accommodate the increased cable tightening due to the substantial coiling and bending of the insertion tube through which the steering cables pass.
An arrangement to keep steering cables as short as possible is described in U.S. Pat. No. 4,794,912. That patent describes a braid-and-bladder pneumatic or hydraulic "muscle," i.e., linear traction motor, that addresses many of the problems found in these prior-art steering mechanisms. Specifically, fluid dynamic muscles mounted adjacent the distal end of the insertion tube are actuated by pneumatic or hydraulic pressure supplied through small flexible tubes within the borescope insertion tube. Short steering cables connect the respective muscles with the articulation mechanism. As fluid pressure is applied differentially to a pair of muscles, the cables move differentially and the articulation mechanism bends the steering section a desired amount.
While this system avoids many of the above-mentioned problems, especially those associated with extremely long cables, there are residual problems because of the reliance on an otherwise conventional cable steering mechanism. The steering section is rather complex and expensive, and does not follow a natural arc, as mentioned before. Further simplification by replacing the cable driven steering mechanism should reduce or eliminate these residual drawbacks, but until very recently suitable alternative steering mechanisms have eluded those in the art.
A fluid bendable steering section for a borescope or endoscope is described in copending U.S. patent application Ser. No. 539,232, filed June 25, 1990, now U.S. Pat. No. 5,018,506, granted May 28, 1991 . In this arrangement, there is an articulation or steering section formed of an elongated tubular elastomeric bladder and a tubular braid disposed over the bladder to confine it so that when the bladder is inflated it expands the braid laterally but shortens it axially. A distal fitting, which can also carry the viewing head, seals the distal end of the bladder and serves as a point for mechanical attachment to the distal end of the braid. A proximal connector sealably fits the proximal end of the bladder and anchors the proximal end of the braid. A central passage through the connector communicates fluid pressure from a controlled fluid pressure source (i.e., pneumatic or hydraulic) to the interior of the bladder for controllably inflating same. A resiliently bendable, but axially incompressible spine is disposed between the bladder and the braid and extends along one side of the longitudinal axis of the bending neck. The spine includes clamping sections at its proximal and distal ends for mechanically affixing the spine to the proximal connector and the distal fitting. When fluid pressure is applied to the interior of the bladder, the braid expands laterally and shortens axially on the unsupported side, i.e., on the side away from the spine. This bends the neck a controlled amount that depends on the fluid pressure applied. The gas or liquid pressure is communicated to the inside of the bending neck through the inside of the insertion tube, which is attached to the proximal end of the proximal connector.
In a preferred mode, the spine is biased, or naturally formed in an arc that is bent to the spine side of the bending neck. When the applied pressure is at a zero or threshold pressure, the neck is biased to that side. When full pressure is applied, the neck is bent arcuately in the opposite direction. At an intermediate pressure, the bending neck is substantially straight. The degree of curvature within the plane of bending is substantially proportional to the applied pressure.
The viewing head, which can be optical i.e. (fiber optic) or video (e.g., a CCD imager) can be situated in the distal fitting of the bending neck. A signal conduit or bundle, which can be a wire bundle in the case of a video device or a fiber optic bundle in the case of an optical device, passes from the head through the distal fitting, then along the axis of the bladder and through a central passage in the proximal connector, and thence through the insertion tube exiting to a suitable viewing device. In either type of probe, a fiber optic bundle is also used to carry illumination to the viewing head for illuminating a target in an enclosed area. A clearance of the passage with respect to the signal conduit and fiber optic bundle can serve to communicate fluid pressure through the insertion tube to the interior of the bladder.
This braid-and-bladder bending neck provides two-way steering (i.e., in a single bending plane) without cables, cable sheaths, or wobble washers. The diameter of the bending neck can be made extremely small, permitting a steerable probe to be constructed of small diameter, e.g. 4 mm. The neck bends substantially along a true arc over its operating range, e.g. from -90 or more to +90 or more degrees of arc.
The biased muscle bending neck deflects because of axial contraction of that part of its braid located radially away from the spine. The wires and illumination fibers are between this axially contracting braid and the axially rigid spine. The wires and fibers are effectively rigid, in the axial direction, and are fixed to the viewing head. Consequently, during deflection of the bending neck both the wires and the fibers must move axially relative to the proximal end of the bending neck. This "pumping" of the wires and fibers must be accommodated somehow in the probe.
In conventional probes, the wires and fibers are free to move axially in and out of the control section when the steering section is articulated or deflated. However, in a biased bending neck probe, where in the inside of the insertion tube is pressurized for steering, the "pumping" problem is more complicated. In a probe of this type, the insertion tube proximal end, wires, and optical fibers are all joined together. This forms a gas seal (or other fluid seal) through which the fibers and wires must pass. The gas seal is necessary to accommodate pressurizing the insertion tube to carry gas or liquid. Therefore, the fibers and wires are not free to accommodate any axial movement where they exit the insertion tube. As a result, the axial tension or compression in the wires and fibers reduces the articulation range of the steering section. Also, the bending neck imposes axial forces on the fiber optic bundle and on the electric wires because they do not stretch and compress with the bending neck. These forces can damage the wires and fibers, especially if either or both should buckle when the force is compressive.